1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to methods and apparatus for providing emergency back-up service for a building when the primary elevator service malfunctions.
2. Description of the Prior Art
Certain types of malfunctions can occur in an elevator system which results in failure of the elevator cars to respond to hall calls, i.e., calls for elevator service placed in the hallways of the floors of a building. For example, the power supply associated with the hall circuits can fail, the group supervisory control or dispatcher which assigns hall calls to the elevator cars can malfunction, or the communication link between the dispatcher and elevator cars can fail. A prior art arrangement for providing emergency back-up service is called "block" operation. The criteria for block operation is to: (a) serve all floors of the building, (b) in the shortest possible time, while (c) making as few stops as possible. Item (c) prevents shortening the motor life due to overheating, and it avoids tripping of the motor overloads which take the elevator car out of service. In the prior art mode of block operation, each car is assigned a predetermined different group of floors, and each car stops at the floors of its group. If a car is out of service, or it goes out of service, its assigned floors would only be served when a car call in an operational car selects one of these floors, resulting in practically no service for these floors. Some prior art systems, as a defense against all cars not being in service, assign more than one elevator car to each floor. This practice, however, significantly increases the time to serve each call, degrading item (b) above, and it increases the number of stops for each elevator car per round trip, degrading item (c).